Propionic acidemia is a disorder of organic acid metabolism in which there is a deficiency of propionyl CoA carboxylase. Propionic acid is an intermediate in the catabolism of several amino acids including isoleucine, methionine, valine and threonine as well as of odd chain fatty acids and cholesterol's side chain. Neonatal propionic acidemia typically presents with metabolic ketacidosis, lethargy, vomiting, hypotonia, and occasionally seizures. The disease course is generally marked by recurrent episodes of metabolic decompensation, almost certain cerebral damage, and historically death in the first decade of life. Efforts aimed at early therapy through limitation of offending amino acids in the diet, carnitine supplementation and vigorous attempts at monitoring metabolic stability and treating metabolic crises have been disappointing in that patients have continued to fare poorly, suffering cerebral atrophy and a plateau in mental functioning during the early years of life. The level of glycine in the blood of patients with propionic acidemia is markedly elevated, though the level of this amino acid in CSF is unclear (it is anticipated to have a significant elevation as well.) The study proposes that the poor neurodevelopmental outcome observed in patients with propionic acidemia may be attributable, at least in part, to the chronic excitotoxic effects of glycine acting at central N-methyl-D-aspartate (NMDA) receptors. The study would evaluate simultaneously the glycine levels in plasma and CSF of 6 patients with propionic acidemia followed by the Metabolism service at CH. If the CSF glycine is found to be elevated, the second phase of the study will use the information to improve the cognitive outcome of patients with propionic acidemia.